Claude Fable 5.1 Decodes 370-Year-Old Historical Cipher in 44 Minutes

An artificial intelligence model has decoded the Cyphral Distich, a 370-year-old cryptographic puzzle authored by Scottish writer Sir Thomas Urquhart in 1653, as first reported by Hacker News. Anthropic’s Claude Fable 5.1 model solved the historical cryptogram in 44 minutes using 176,000 tokens, requiring zero human intervention during the automated execution process.

The cipher appears at the end of Urquhart’s book Logopandecteision and consists of two lines containing 32 numbers each. The text remained unsolved for centuries despite being published as an open challenge in Notes and Queries in 1899 and appearing in 20th-century cryptographic literature. Historical cipher researcher Klaus Schmeh previously listed the cryptogram among his top 50 unsolved encrypted messages. Early decipherment efforts failed because cryptanalysts relied on traditional methods such as frequency analysis, simple substitution, and homophonic substitution, assuming the key was an external cipher alphabet.

Fable 5.1 cracked the code after identifying that the book itself served as the decryption key. The model observed that the cryptogram was printed directly following Urquhart’s "32 Proquiritations," a section where the author explicitly wrote that "there can no number like that of two and thirty … be pitched upon." Additionally, an accompanying poem noted that attentive readers would discover the author's desires within the text, matching the phrasing used throughout the Proquiritations.

By linking the cipher's structure to the text, the model established a direct indexing rule: for the i-th number in a cipher line, navigate to the i-th Proquiritation, locate that word index, and take its first letter. This process generated a rhyming 32-letter two-line verse: "O GOD UPHOLD KING CHARLS THE SECOND AND MAKE HIM THE SUPREME RULER OF THIS LAND." The decoded message reflects Urquhart’s historical ties as a dedicated Royalist during the English Civil War era.

Following its initial breakthrough, Fable 5.1 applied the same book-based indexing technique to solve a second cryptogram by Urquhart. The model successfully decoded the Cyphral Octastich from Urquhart’s 1652 work The Jewel, a considerably larger puzzle comprising 285 numbers that had similarly eluded cryptanalysts for centuries.

The experiment was designed by prompting the AI model to locate and solve an authentic, verifiable historical cipher. The researcher instructed Fable 5.1 to avoid ambiguous puzzles with multiple plausible answers as well as notoriously complex codes like Kryptos K4. Guided by these boundaries and encouraged to evaluate its own problem-solving progress, the model identified Urquhart’s cipher as uniquely tractable after recognizing the numerical alignment between the code and the book’s structure.

The successful decryption demonstrates the potential of large language models to resolve archival mysteries and historical conjectures that were previously bottlenecked by human attention. Rather than relying on advanced mathematical cryptanalysis, Fable 5.1 succeeded by systematically cross-referencing contextual hints, demonstrating that persistent information retrieval across obscure literature can yield solutions to long-standing historical puzzles.